I don't have a lot to add thus far that Frank C. hasn't shared that is most accurate in context to your challenge.
What I would add is more an observation of current trends in methods trying to replicate such project with...what again...once was traditional woodworking methods. The choice then is up to you to choose the modality you wish to practice or afford yourself of.
I see potential wonderful project like this gate being "reinvented" to fit the modern context, concepts, and approaches in woodworking...which seldom (if ever?) are the most enduring or actually applicable. These modern solution sometimes work (or work for a while...at least) yet are often just "dumb-down" and/or industrial approaches to traditional methods...modern in nature and condition yet seldom better (or more effective?) solutions to the original (aka traditional context) of a given task or the methods to achieve it.
What I can share, that does give me pause from your most recent post and photo is the lack of proper grain slope for your gate top, or mitigation methods to counter potentials in grain slip and checking. This is perhaps one of the more common mistakes (for lack of better description?) in creating what once would have been achieved by properly selecting a piece of timber that more dynamically fits the topography and couture of the desire shape...or...jointing several germane piece together...or...bending work. Thus, matching in grain slop and/or contours so to be fit the shape and not to distort and/or disfigure over time. Such outdoor frameworks in wood are subject to many extremes in not only thermal flux seasonally yet also extreme moisture swings, and why many (most?) of such work is approached more like a traditional timber frame (aka green wood) since it will never really be "dry" within the modern contextual practice. As such, the finishes used today only exacerbate the rapid breakdown to the woods interstitial structure because, once again, the traditional finishing modalities are abandoned for "plastic" film finishes that do little more than either trap moisture there by accelerating decay of the poorly selected wood.
In closing I would offer looking at the oldest versions of such gates (over 3 centuries or more)...how they are created...what species...in what methods as it relates to joinery...and compare them to the longest lived gates we see today in the modern context which is maybe 30 to 40 years for the best of them...
Food for thought and consideration as you proceed, or for future projects...
Last edited by 35015; 07-28-2019 at 04:56 PM.